Shock absorbing devices are used in a wide variety of vehicle suspension systems for controlling motion of the vehicle and its tires with respect to the ground and for reducing transmission of transient forces from the ground to the vehicle. Shock absorbing struts are a common and necessary component in most aircraft landing gear assemblies. The shock struts used in the landing gear of aircraft generally are subject to more demanding performance requirements than most if not all ground vehicle shock absorbers. In particular, shock struts must control motion of the landing gear, and absorb and damp loads imposed on the gear during landing, taxiing and takeoff.
A shock strut generally accomplishes these functions by compressing a fluid within a sealed chamber formed by hollow telescoping cylinders. The fluid generally includes both a gas and a liquid, such as hydraulic fluid or oil. One type of shock strut generally utilizes an “air-over-oil” arrangement wherein a trapped volume of gas is compressed as the shock strut is axially compressed, and a volume of oil is metered through an orifice. The gas acts as an energy storage device, such as a spring, so that upon termination of a compressing force the shock strut returns to its original length. Shock struts also dissipate energy by passing the oil through the orifice so that as the shock absorber is compressed or extended, its rate of motion is limited by the damping action from the interaction of the orifice and the oil.
Over time the gas and/or oil may leak from the telescoping cylinders and cause a change in the performance characteristics of the strut. Presently, there is no reliable method of verifying the correct servicing parameters of aircraft shock struts. While gas pressure can be readily monitored, it cannot be readily determined if a loss in gas pressure arose from leakage of gas alone or from leakage of both gas and oil, unless external evidence of an oil leak is noticed by maintenance personnel. If a low pressure condition is detected in the absence of external evidence of an oil leak, maintenance personnel heretofore would restore the gas pressure to a prescribed level by adding gas. This, however, eventually leads to degraded performance of the shock strut if oil had indeed escaped from the strut. Even if evidence of a oil leak is observed, maintenance personnel cannot easily determine how much oil remains or whether the remaining amount of oil meets specifications or is acceptable for operation.
Two methods can be used to determine whether a strut has the correct pneumatic charge. One method is to jack-up the aircraft to take the weight off of the struts such that each strut is fully extended. The proper pressure that corresponds to the extended position of the strut is a known value. In the other method the pressure is measured with the aircraft supported by the strut using a pressure gauge, and the stroke is measured to determine the extension of the strut. Variations in the weight of the aircraft and the position of the center of gravity cause the strut to sit at a variety of strokes in this situation. A look-up table or chart is then used to verify that the stroke and the pressure match an acceptable value. Since jacking the aircraft is rarely done and is very time consuming, the method of verifying the pressure with the aircraft supported by the strut in a static position is most commonly used. This latter technique, however, is not a very reliable way to check the oil level.
The only reliable way to know that the oil level is acceptable is to vent the pneumatic charge and pump oil through the strut to ensure a proper oil level. The strut can then be re-inflated with gas to the proper pressure. This operation takes a significant amount of time, and as a result maintenance personnel may skip this step and only correct the pressure by adding or venting gas. In addition, neither technique enables detection of the oil level while the aircraft is in flight.